md: avoid spinlock problem in blk_throtl_exit

blk_throtl_exit assumes that ->queue_lock still exists,
so make sure that it does.
To do this, we stop redirecting ->queue_lock to conf->device_lock
and leave it pointing where it is initialised - __queue_lock.

As the blk_plug functions check the ->queue_lock is held, we now
take that spin_lock explicitly around the plug functions.  We don't
need the locking, just the warning removal.

This is needed for any kernel with the blk_throtl code, which is
which is 2.6.37 and later.

Cc: stable@kernel.org
Signed-off-by: NeilBrown <neilb@suse.de>

drivers/md/raid5.c

@@ -5204,7 +5204,6 @@ static int run(mddev_t *mddev)
 
 		mddev->queue->backing_dev_info.congested_data = mddev;
 		mddev->queue->backing_dev_info.congested_fn = raid5_congested;
-		mddev->queue->queue_lock = &conf->device_lock;
 		mddev->queue->unplug_fn = raid5_unplug_queue;
 
 		chunk_size = mddev->chunk_sectors << 9;